Ceramic transducers having annular elements



Jan. 31, 1956 L. w. CAMP 2,733,423

CERAMIC TRANSDUCERS HAVING ANNULAR ELEMENTS Filed Feb. 18, 1952 INVENTOR.

Leon W. Camp BY ATTORNEY 33 CEIC TRANSDUCERS, HAVING ANNULAR ELEMENTS Leon W. Camp, Glendale, Califi, assignor to Bendix Aviation Corporation, North Hollywood, (Ialifi, a corporation of Delaware Application February 18, 1952, fierialNc. 272,207 6 Claims. (Cl. 340-) This invention relates to transducers for converting electrical oscillations intotraveling compressional waves in fluids and vice versa, and has to do particularly with transducers for this purpose employing electro-mechanically responsive ceramics such as barium titanate.

These ceramics resemble piezo electric crystals in that h y e m ca l n. sn w e es risa p en and vice versa, but have the great advantage over piezo electric crystals in that they are not so limited asto shape or size, are very rugged, and are relativelyelficient converters of energy from electrical .to mechanical form and vice versa. t l I This is a continuation in part of my application Serial No. 214,144, filed March 6, 195 1, non/ abandoned and titled Ceramic Transducers for Underwater Sound Transmission and Reception.

An object of the inv ntion is to provide ceramic transducer designs utilizing the special propertiesof electromechanically responsive ceramics to high advantage in converting traveling waves in fluids to electrical oscillations and vice wersa.

Other more specific objects and features of the invention will appear from the following description with reference to the drawing, in which;

Fig. 1 is a longitudinal sectional view of a transducer employing a short annular. ceramic element.

Fig. 2 is a view similar to Fig. 1 showing a variation of the structure shown in Fig, 1

Fig. 3 is a view similar toFig. 2 showing a modified construction; and i Fig. 4 is a cross of Fig. 3.

The present invention is directednct. to any particular ceramic composition, but to shapes and methods of mounting ceramic elements to obtain efiicient transducers. Various compositions have been developed and are being developed, but for the purpose of thep resent invention it is merely necessary hat t e mate ia be le c a i y e po s e nd be epsbls b in f r into the shapes disclosed. Such shaping may done by molding or tr n the mat r al Ii l ti hf u P O s io aken n the P ea 1V IV firing, or by cutting or machining atter firing. Thelmaterial is polarized after, firing by applying unidirectional electric' field in the direction of 'desired polarity, which is betweenthe working electrodes, so that the latter can be used to apply t e polariz ng potential.

Barium titanate is a common basic ingredient of presently kncwn ceramic electro-mechanically respcnsive elements, and for disclosures of compositions of some electro-mechanically responsive ceramics'that maybe used, reference is made to Wainer'Patents 2,402,515, 2,402,516 and 2,402,518, but the present invention is in no sense limited to those particular compositions. For convenience, electromechanically responsive elements capable of being formed into the shapes disclosed herein will be referred to hereinafter simply as ceramics.

It is characteristic of presently known ceramics that the direction of the primary mechanical forces produced by ited States Patent an electrical field therein is parallel to the field, in contrast to piezo-electric crystals in which the primary mechanical force is perpendicular to the field in most sonic and supersonic applications,- The present invention utilizes this characteristieof ceramics to full advantage.

The transducers herein disclosed are particularly adapted for the transmission or reception of supersonic vibrations in water for use in depth sounding, underwater ranging, etc., but since they are not limited to any particular frequency range, thevibrations' will be referred to, for convenience, as sound, which term as used herein includes both the sonic and supersonic range of frequencies Fig. 1 shows a transducer consisting of a fluid-tight case 20 closed at the front end bya' sound transparent window consisting of, a cap Zlof a rubber having sub stantially the same sound-transmittingproperties as the water or other iluid in which the transducer is to be immersed, so that it transmits sound with substantially no reflection. The cap 21 is shown secured in place by a band clamp 23.

There is contained within the case 20 an electro-mechanically responsive radially vibratile annular transducing element consisting of a ceramic ring'24 havinginner and outer electrodes 24a and 24b respectively, which may consist of thin silver films bonded to the internal and external surfaces of the ring. The ring 24 is backed by a solid member 25 of substantial size, which may be made of some material such as steel or aluminum having good sound transmission characteristics and a specific acoustic impedance much greater than that of water. The diameter of the backing member 25 is substantially the same as that of the ceramic ring 24, and both'ar'e supported within the casing 20 by'a mass of some sound in- 'sulating materialZd, such as Corprene or air-cell rubber. The entire space between the front end of the member 25 and the window 21 is filled with some'fiuid suchas castor oil, which has substantially the same sound propagation characteristics as water and propagates sound between the window 21am: the annular inner surface of the ring (the surface to which the electrode 240 is atc gdy The direct or primary mechanical motion produced in the ceramic ring 2453 potential between the electrodes 24a and 24b is ra dial, but there is a secondary circumferential movement which determines the'reso nant fre quen cy, it being approximately equal to one Wave length of sound in the material at the resonant frequency. The radial vibration of the outer face of the ceramic ring 24 is not utilized, it being reflected by the sound insulating material 26. The radial vibration of the inner surface of the ring 24 is transmitted toth fiuidtherewithin, developing sound waves which are transmitted through the fluid to the "window 21, and throughthe'window to the water or other fluid exterior thereof.

A ceramic ring 24 suitabletor operation at 37.5 kilocycles may have an axial dimension of'one centimeter, a radial thickness of 3,2'c'entimeteraan'd a mean diameter Qqp l t r. l

where is the wave length of sound in the ceramic at the stated frequency. The lengtli o'f'the backing member 25 (if of steel) may be 3.3 centimeters, which'i's approximately one-fourth of the wave length of sound steel.

Fig. 2 shows a modification of the structure shown in Fig. 1, in which the backing member 27 is made of larger diameter, and has an integral forward extension defining a plurality of recesses 27a to receive a plurality of radially vibratile annular transdncing elements consisting of ceramic rings 24. The recesses 27a are preferably of substantially the same depth as the rings 24 so that when the bases of the rings are resting against the bottom of their recesses, their outer faces are approximately flush with the front face of the backing member 27. The base portion of member 27 rearward of the recesses 27a constitutes a solid sound-reflective wall member, and the portions forward of the base portion constitute an integral forward extension thereof for supporting the rings 24.

Any desired number of rings may be employed in a single transducer, as shown in Fig. 2. This multiple construction is not limited to the particular design of Fig. 2, but can also be applied to that of Fig. 1.

However, the use of the block 27 having the recesses 27a for receiving the ceramic rings 24 is particularly advantageous for multiple unit transducers, in that practically all surfaces exposed to the oil that fills the space back of the rubber cap 21 are sound reflective. This substantially increases the efficiency. Tests on comparable multiple units corresponding to Figs. 1 and 2 respectively have shown an efficiency approximately 3 /2 db higher for the construction of Fig. 2 than for that of Fig. 1.

To permit radial movement of the outer face of each ceramic ring 24, in the construction of Fig. 2, the internal diameter of the recess in which the ring fits is made somewhat greater than the outside diameter of the ring, and the ceramic ring is supported clear of the recess wall in sound insulated relation therewith by a ring 28 of some sound absorptive insulating material such as air-cell rubber.

There is shown in Figs. 3 and 4 a modication of the structure shown in Fig. 2. In the construction of Fig. 3 r

the case consists of a front cylindrical case 30 and a rear closure cap 31 secured together in fluid tight relation by means of a gasket 32 and screws 33. The front wall member or sound window 34 is set into a counterbore 35 in the front end of the case 30 and bonded thereto. A

fiat faced solid sound-reflective wall member or plate 36 is mounted in the case 30 with its rear face flush with the rear face of case 30. Plate 36 is supported at its peripheral face from the case 30 by a ring 37 of Corprene or the like. The Corprene ring 37 and case 30 constitute means joining the solid wall member or plate 36 and the front wall member 34 together at their edges to form an enclosure containing the ceramic rings 24. The peripheral portion of the closure cap 31 is thicker than the case 30, so that it extends inwardly to engage the rear face of the plate 36 and support it against rearward movement.

These ceramic rings 24 are positioned in apertures 38a in a disc 38 of air cell rubber or Corprene, and the assembly of disc 38 and rings 24 is sandwiched between the sound window 34 and the plate 36. Leads 40 from the electrodes of the ceramic rings are brought back through holes 36a in the plate 36 to a cable 41 which enters the cap 31 through a neck 31a thereon. The cable is sealed by a packing ring 42 compressed by a packing gland 43, and by a potting compound 44 poured into the neck 31a. All free space within the case is filled with a suitable oil, which is supplied through a filler opening 45 in the cap 31.

Whereas the rings 24 are shown spaced from the sound window in Fig. 2, they are shown in contact therewith in Fig. 3. However, this does not appear to affect the efficiency of the transducer, and from a manufacturing standpoint the structure of Fig. 3 is more practical than that of Fig. 2. It is to be understood that at least a film of oil will separate the inner surface of the sound window 34 from the disc 38 and the front surfaces of the ceramic rings 24. Also, suificient oil may be introduced to slightly bulge the window 34 away from the disc 38 except at the edges of the latter.

Although for the purpose of explaining the invention, a particular embodiment thereof has been shown and described, obvious modifications will occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

I claim:

1. In a transducer for translating sound waves in a fluid medium into electrical waves in an electric circuit, and vice versa: a front wall member of sound-transparent material; a solid wall memberof sound-reflective material back of said front wall member in generally parallel relation thereto and substantially coextensive therewith, and means joining said wall members together at their edges to form an enclosure; an array of radially vibratile annular transducing elements positioned in said enclosure, each element resting at one end against said solid wall member; means including sound-absorptive material interposed between said elements for supporting them in separated relation and preventing effective acoustic cou pling between their outer surfaces and said front wall member; a sound-propagating compressional wave medium filling the free space within said enclosure for propagating sound between said front wall member and the annular inner surfaces of said elements; and electrical means associating said elements with said electric circuit.

2. A transducer according to claim 1 in which said means interposed between said elements comprises an integral forward extension on said solid wall member defining cylindrical recesses in which said elements are individually positioned, said recesses being of larger diameter than said elements, and a ring of sound-absorptive material separating the said outer surface of each element from the inner surface of its recesses.

3. A transducer according to claim 2 in which the depth of said recesses is substantially the same as that of said elements.

4. A transducer according to claim 1 in which said means interposed between said elements comprises a sheet of sound-absorptive insulating material substantially coextensive with and lying against said solid wall member and of thickness substantially equal to the axial thickness of said elements, said sheet having cylindrical apertures extending therethrough individually containing said elements.

5. A transducer according to claim 4 in which said front wall member is substantially in contact with said elements and said sheet.

6. A transducer according to claim 5 in which said means joining said wall members together at their edges comprises a peripheral case secured to and extending rearwardly from the edge of said front wall member and dimensioned internally to loosely receive therewithin the peripheral face of said solid wall member; a layer of solid sound-absorptive material filling the space between the peripheral face of said solid wall member and the inner face of said peripheral case; and a closure cap secured to and closing the rear end of said peripheral case, said closure cap having a shoulder engaging the rear face of said solid wall member for preventing rearward movement thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,391 Mason July 23, 1946 2,405,187 Benioif Aug. 6, 1946 2,473,354 Beniofi June 14, 1949 2,497,901 Mott Feb. 21, 1950 

1. IN A TRANSDUCER FOR TRANSLATING SOUND WAVES IN A FLUID MEDIUM INTO ELECTRICAL WAVES IN AN ELECTRIC CIRCUIT, AND VICE VERSA: A FRONT WALL MEMBER OF SOUND-TRANSPARENT MATERIAL; A SOLID WALL MEMBER OF SOUND-REFLECTIVE MATERIAL BACK OF SAID FRONT WALL MEMBER IN GENERALLY PARALLEL RELATION THERETO AND SUBSTANTIALLY COEXTENSIVE THEREWITH, AND MEANS JOINING SAID WALL MEMBERS TOGETHER AT THEIR EDGES TO FORM AN ENCLOSURE; AN ARRAY OF RADIALLY VIBRATILE ANNULAR TRANSDUCING ELEMENTS POSITIONED IN SAID ENCLOSURE EACH ELEMENT RESTING AT ONE END AGAINST SAID SOLID WALL MEMBER; MEANS INCLUDING SOUND-ABSORPTIVE MATERIAL INTERPOSED BETWEEN SAID ELEMENTS FOR SUPPORTING THEM IN SEPARATED RELATION AND PREVENTING EFFECTIVE ACOUSTIC VOUPLING BETWEEN THEIR OUTER SURFACES AND SAID FRONT WALL MEMBER; A SOUND-PROPAGATING COMPRESSIONAL WAVE MEDIUM FILLING THE FREE SPACE WITHIN SAID ENCLOSURE FOR PRPAGATING SOUND BETWEEN SAID FRONT IALL MEMBER AND MEANS ASSOCIATING SAID ELEMENTS WITH SAID ELECTRIC CIRCUIT. 